Scanning probe microscopy (SPM) devices, such as atomic force microscopy (AFM) devices as described above are for example applied in the semiconductor industry for scanning of semiconductor topologies on a surface. Other uses of this technology are found in biomedical industry, nanotechnology, and scientific applications. In particular, AFM may be used for critical defect metrology (CD-metrology), particle scanning, stress- and roughness measurements. AFM microscopy allows visualization of surfaces at very high accuracy, enabling visualization of surface elements at sub-nanometer resolution. Other surface scanning measurement devices for example include optical near field scanning devices.
The probe in an SPM system comprises a cantilever and a probe tip. On one end of the cantilever, the probe is attached to a sensor head, for example (but not necessarily) through an actuator that allows to bring the probe in motion. Probe tip is usually located on the other end of the cantilever. In SPM, the probe tip is scanned over the surface of a sample or substrate to measure the topography and mechanical properties thereof. A sensor, in many cases an optical sensor, monitors the position of the probe tip. For example, the sensor may monitor a reflected laser beam that is reflected by the cantilever or the back of the probe tip, and which changes angle when the probe tip moves up or down.
Depending on the circumstances and the type of information desired, SPM may be performed in various modes of operation. These modes include static modes, wherein the cantilever is held static while the probe tip moves perpendicular to the surface due to surface features that are encountered during scanning. Dynamic operation modes are another important class of modes, wherein the cantilever is vibrated during scanning thereof across the surface. Generally, the topography image of the surface is obtained by measuring the effects of the surface on the probe and keeping it constant by adjusting the height of the probe using a feed back loop and a piezoelectric actuator. The height of the probe is then monitored, and from this, the topography can be obtained. A number of different modes are available and known to the skilled person. For AFM, these modes for example include tapping mode AFM (TM-AFM) wherein the probe is brought intermittently in contact with the surface and which may performed in combination with amplitude modulation AFM (AM-AFM) wherein the amplitude is kept constant, frequency modulation AFM (FM-AFM) wherein the resonance frequency of the probe is kept constant, and peak force tapping mode AFM (PFT-AFM) wherein the probe is moved quasi-statically and maximum deflection is kept constant. In general, it is important to keep the forces constant during the scanning, however it is not possible to measure these forces with the methods applied.
One of the most used AFM operation schemes is Tapping Mode AFM (TM-AFM). In this technique the cantilever is excited with a frequency near its fundamental resonance frequency and brought in intermittent contact with the sample surface until its amplitude decreases to a certain set point amount. While scanning, the amplitude is kept constant by adjusting the distance between the cantilever and the sample. The control signal amplitude error and phase signals are interpreted as the height, amplitude and phase images, respectively. In TM-AFM the tip hits the sample surface in every cycle, and thus experiences both attractive and repulsive forces. However, since these forces occur only during a short fraction of a tapping cycle, the cantilever only responds to the periodic average of the forces. Consequently, the motion of the cantilever remains harmonic and its amplitude and phase evolve only with the periodic average of the forces. Since only the average of the forces affect the cantilever, and countless different types of forces can have the same average, it is impossible to fully estimate the forces from TM-AFM signals.